A treble flute is a flute in G, I believe, per Wikipedia. It’s not in the brass family: it’s in the woodwind family, not only in the real world but in Dorico too. I guess you probably typed “tr” and saw “trumpet”, “trombone”, “treble flute”, etc., and assumed that all of the entries in the middle column of the picker were from the same family, whereas if you select a different entry in the middle list, you’ll see that in fact the correct family in the left-hand list is highlighted as you move down the middle list.
Dorico doesn’t have the old “Piccolo in Db” formerly used in military bands (and found in some old editions of Sousa marches). It’s probably obsolete now, but 5 flats is a good key for brass instruments.